Fix bug in _SAT collision test

The test improperly detected min/max values in some cases.  This fixes that issue by comparing to +/- Infinity instead of special casing the initial values.  (It also gets rid of a now needless comparison, which might help performance.)

This also includes a test that catches the issue.

Fixes issue #828
Merge pull request #829 from starwed/sat-fix

Fix bug in _SAT collision test